Today, the vast majority of electric energy supplied to end users is transmitted as alternating current or AC. The reasons for this are somewhat historical: in the early days of the grid there were no power electronics and one of the main advantages of AC was that it could easily be converted to high voltage for transmission (at reduced loss), and then reconverted to low voltage for the end user (for increased safety).
There is, however, a growing movement across the energy industry to start using direct current (DC) networks instead of AC networks. There are many strong arguments in favor of this: today's power electronics mean that the original advantages of AC are no longer relevant; at very high voltages, DC transmission is actually more efficient than AC; and at low voltages, DC networks are easier to maintain and have fewer problems (e.g., no harmonics).
However, perhaps the most important argument in favor of DC networks is that the majority of end user loads and sources are actually already DC-based. What is commonly done today is to take locally generated DC power (e.g., from photovoltaic panels) and convert it to AC using inverters. Most of the time, however, the power supply in the end user equipment simply converts the AC it receives back to DC. See, for example, Singh et al., “DC Microgrids and the Virtues of Local Electricity,” IEEE Spectrum (posted February 2014).
At the same time, there is a large movement throughout the energy industry towards distributed networks and microgrids. Conventionally, microgrids have used sources like diesel generators to generate AC power. More and more, these applications find it cost effective to instead use a combination of renewables (e.g., solar power) and energy storage to provide their energy needs. These kinds of sources are much better suited to DC.
One challenge in the operation of DC networks, however, is to ensure that there is proper power sharing between sources. Namely, many microgrids will have multiple batteries (or generation sites). If these are at different distances (i.e., different impedances) to the loads that they supply, then due to the nature of electrical networks there will be an uneven amount of power being supplied by different sources. For instance, if a first source and a second source have a distance L1 and L2 respectively to a load, and L2>L1, then due to a greater distance to the load/greater impedance, more power will be drawn from the first source to power the load.
If power sharing between sources is not addressed, then the possible risks/disadvantages involve: decreased stability of the microgrid; uneven discharge (and charge) of energy storage devices; higher rates of aging and replacement for energy storage or distributed generation devices; and unpredictable asset replacement schedules.
Active power sharing problems in DC microgrids are in some ways analogous to the reactive power sharing problem in AC microgrids, as shown in Schiffer et al., “A Consensus-Based Distributed Voltage Control for Reactive Power Sharing in Microgrids,” 13th European Control Conference (ECC), pgs. 1299-1305, Strasbourg, France (June 2014). However, most existing methods use external communication infrastructure which can be costly and can introduce vulnerabilities and complexity.
Therefore, there is a strong need for a stable, simple solution to ensure fair power sharing between multiple energy sources in DC networks.